1. Field of the Invention
The present invention relates to a transformer and a manufacturing method thereof. More particularly, the present invention relates to a high coupling factor transformer and a manufacturing method thereof.
2. Description of Related Art
Some application circuits, such as radio frequency circuits, voltage controlled oscillator circuits, and mixer circuits, generally have one or more transformers. With the present trend developing lighter, thinner, shorter and smaller electronic products, transformers in an electronic circuit are usually implemented in an integrated circuit. In the application of a transformer, the coupling factor k is quite an important parameter in circuit design. A high k value means a high energy (or signal) conversion rate, which also means that the energy (or signal) loss can be reduced.
FIG. 7 illustrates the layout of a general transformer. Referring to FIG. 7, the conventional transformer 700 is a planar transformer. Generally speaking, in order to avoid the coupling effect between the transformer and the devices of the IC, the transformer is usually disposed far from the substrate. For example, if eight metal layers are composed in the manufacturing of the transformer 700, a primary winding 710 and a secondary winding 720 are normally disposed at the eighth metal layer M8. FIG. 8A illustrates the characteristic of the coupling factor k of the transformer in FIG. 7. FIG. 8A is a measurement result of the frequency ranged at 0˜20 GHz obtained from the primary winding 710 and the secondary winding 720 of the transformer. The outer diameter of the transformer 700 is 250 um, and the width of the windings and the distance between the windings are respectively 9 um and 1.6 um. FIG. 8B illustrates the characteristic of the coupling factor k of the transformer in FIG. 8A when the frequency is ranged between 0˜3.5 GHz. It is clear from FIG. 8B that the coupling factor k is about 0.745 when the working frequency of the transformer 700 is 1 GHz.
Furthermore, other various types of transformers have been disclosed in different documents for improving the coupling factor k value. For example, the implementations of various transformers are disclosed in Implementation of High-Coupling and Broadband Transformer in RFCMOS Technology published in Journal of Solid-State Circuit, Vol. 52, No. 7, page 1410-1414, July 2005, Stacked Inductors and Transformers in CMOS Technology published in Journal of Solid-State Circuit, Vol. 36, No. 4, page 620-628, Apr. 2001, Ultra-Low-Voltage High-Performance CMOS VCOs Using Transformers Feedback published in Journal of Solid-State Circuit, Vol. 40, No. 3, page 652-660, Mar. 2005, 11-GHz CMOS Differential VCO With Back-Gate Transformer Feedback published in Microwave and Wireless Components Letters, Vol. 15, No. 11, page 733-735, Nov. 2005, A 1-V Transformer-Feedback Low-Noise Amplifier for 5-GHz Wireless LAN in 0.18-μm CMOS published in Journal of Solid-State Circuit, Vol. 38, No. 3, page 427-435, Mar. 2003, of the Institute of Electrical and Electronic Engineers (IEEE); Paper No. FA8.6, entitled A Fully Integrated CMOS 900 MHz LNA utilizing Monolithic Transformers, of the International Solid-State Circuits Conference (ISSCC98); and U.S. Patent Publication No. 4,816,784, U.S. Patent Publication No. 6,577,219, U.S. Patent Publication No. 6,608,364, and U.S. Patent Publication No. 6,927,664.